PET and SPECT Ligands for Imaging Neuronal Apoptosis Abstract: Activation of the intracellular caspase cascade is a well understood signaling mechanism that results in apoptosis. The identification of apoptotic events in vivo has important applications for detecting cell death induced by disease, injury, therapeutics, or normal tissue development and differentiation. Further advances in these lines of investigation increasingly call for in vivo methods and probes for detection and imaging of intracellular signaling events. This 2 year Phase I STTR application proposes to conjugate a radiolabel to the pan caspase inhibitor VAD-FMK. This probe will provide a new tool for the non-invasive imaging of caspase-mediated apoptotic events in vivo using PET or SPECT imaging. In the first year of the proposal, eight VAD- FMK analogs will be synthesized to devise the optimal reagent probe. The newly synthesized probes will be evaluated for their ability to detect caspase activity in vitro. By the end of the first year, we will select the four best candidates to radiolabel for the next stage of assessing their pharmacodynamics/pharmacokinetics in normal mice. Finally, we will utilize a mouse model of the neurodegenerative disease multiple sclerosis to evaluate the best two candidate probes for their suitability as a non-invasive in vivo apoptosis detection reagent. Accordingly, the Specific Aims of this proposal are: I. Synthesis of non-radioactive VAD-FMK compounds and validation of their caspase-binding activity, II. Radiolabeling of the candidate probes and measurements of in vivo uptake and biodistribution in normal mice, and III. Evaluation of RC-VAD-FMKs as in vivo detectors of apoptosis in EAE. It is expected that completion of these three specific aims will lead to a Phase II STTR that will proceed into pre-clinical development of the prototypical radiotracer for in vivo detection of neuronal apoptosis. PUBLIC HEALTH RELEVANCE: Cells in the nervous system may die as a result of disease, radiation damage, chemotherapy, or normal aging. This project will produce a probe that can detect cell death inside a living organism. This new tool will help researchers understand how cell death occurs and may suggest new treatments or preventive measures for protecting against neuronal cell death.